This invention relates to an adaptive array antenna receiving apparatus and a method therefor and, in particular, to an adaptive array antenna receiving system in which a transmitted signal of a COMA system is received by a plurality of antenna elements forming an adaptive array antenna.
A CDMA (Code Division Multiple Access) system attracts attention as a radio transmitting system capable of considerably increasing a subscriber capacity. For example, a CDMA adaptive array antenna receiving apparatus used in the CDMA system is disclosed in Wang et at “Adaptive Array Antenna Combined with Tapped Delay Line Using Processing Gain for Direct-Sequence/Spread-Spectrum Multiple Access System” (IEICE Transactions, Vol. J75-BII, No. 11, pp. 815–825, 1992) and Tanaka et al “The Performance of Decision-Directed Coherent Adaptive Diversity in DS-CDMA Reverse Link” (IEICE, Technical Report on Radio Communication System, RCS96-102, November 1996). As disclosed in these articles, an antenna weight is controlled by the use of a weighting control error signal derived after despreading. In this manner, adaptive control is carried out so that an antenna directive pattern maximizing a received SIR (Signal to Interference Ratio) is formed to cancel an interference.
Referring to FIG. 1, description will be made of a related CDMA adaptive array antenna receiving apparatus in case where a common error signal is used. It is assumed here that the number of receiving antennas is equal to N (N being an integer not smaller than 2) and that the number of paths of a multipath is equal to L (L being an integer not smaller than 1). Consideration will be made about the k-th user (k being an integer greater than 1).
As illustrated in FIG. 1, the CDMA adaptive array antenna receiving apparatus comprises N receiving antennas 1-1 through 1-N forming an array antenna, (L-1) delay units 2-2 through 2-L corresponding to the second through the L-th paths of the multipath except the first path of the multipath, respectively, (N×L) despreading circuits 3-1-1 through 3-L-N corresponding to the first through the L-th paths of the multipath and the N receiving antennas 1-1 through 1-N, L antenna weighting/combining circuits 4-1 through 4-L, and a MMSE (Minimum Mean Square Error) control circuit 5 connected in common to the L antenna weighting/combining circuits 4-1 through 4-L, a reference signal producing circuit 7, an adder 6 E and a subtractor 8.
The N receiving antennas 1-1 through 1-N are arranged in close proximity to one another so that a plurality of the received signals are mutually correlated. The delay units 2-2 through 2-L serve to delay the signals propagated through the second through the L-th paths of the multipath and received by the N receiving antennas 1-1 through 1-N. The received signals are classified into the first through the L-th multipath received signals due to delay times in the first through the L-th paths of the multi path. The second through the L-th multipath received signals are supplied to the delay units 2-2 through 2-L, respectively, while the first multipath received signals are directly supplied to the despreading circuits 3-1-1 through 3-1-N. The delay units 2-2 through 2-L delay the second through the L-th muripath received signals in synchronism with the timing on the first path of the multipath to produce second through L-th delayed signals, Therefore, a delay unit 2-1 corresponding to the first path of the multipath is omitted in FIG. 1 because no delay is required.
The despreading circuits 3-1-1 through 3-1-N for the first path of the multipath are directly supplied with the signals received by the receiving antennas 1-1 through 1-N, respectively. The despreading circuits 3-2-1 through 3-2-N for the second path of the multipath are supplied with the second delayed signals produced by the delay unit 2-2. The despreading circuits 3-L-1 through 3-L-N for the L-th-path of the multipath are supplied with the L-th delayed signals produced by the delay unit 2-L. Thus, the despreading circuits 3-l-1 through 3-l-N for the l-th (l=2 to L) path of the multipath are supplied with the l-th delayed signals produced by the delay unit 2-l.
Supplied with the first multipath received signals directly from the receiving antennas 1-1 through 1-N and supplied with the second through the L-th delayed signals from the delay units 2-2 through 2-L, the despreading circuits 3-1-1 through 3-L-N produce despread signals. The despreading circuits 3 1-1 through 3-L-N send the despread signals to the antenna weighting/combining circuits 4-1 through 4-L and to the MMSE control circuit 5. Supplied with the despread signals, the antenna weighting/combining circuits 4-1 through 4-L produce weighted and combined signals. The adder 6 sums the outputs of the antenna weighting/combining circuits 4-1 through 4-L to produce a sum signal as a rake combined signal and supplies the rake combined signal to the subtractor 8.
Temporarily referring to FIG. 2, description will be made of the antenna weighting/combining circuits 4-1 through 4-L. Since the antenna weighting/combining circuits 4-1 through 4-L have the same structure, the antenna weightinglcombining circuit 4-1 alone will be described by way of example. As illustrated in FIG. 2, the antenna weighting/combining circuit 4-1 comprises a plurality of multipliers 9-1 through 9-N and an adder 10.
As described above, the antenna weightingicombining circuit 4-1 is supplied with the despread signals despread by the despreading circuits 3-1-1 through 3-1-N. Supplied with the despread signals and antenna weights produced by the MMSE control circuit 5, the multipliers 9-1 through 9-N multiply the despread signals by the antenna weights to produce weighted signals. The adder 10 sums the weighted signals to produce the sum of the weighted signals as an antenna combined signal and supplies the antenna combined signal to the adder 6 in FIG. 1. By controlling amplitudes and phases of the signals received by the receiving antennas 1-1 through 1-N, the antenna weightingicombining circuits 4-1 through 4-L form the directive pattern of the array antenna so that a desired signal component is given a gain and interference signal components are suppressed.
The adder 6 sums the output signals of the antenna weighting/combining circuits 4-1 through 4-L to produce the rake combined signal. Thus, rake combination is carried out. Supplied with the rake combined signal produced by the adder 6 and a reference signal produced by the reference signal producing circuit 7, the subtractor 8 subtracts the rake combined signal from the reference signal to obtain a common error signal. The subtractor 8 supplies the common error signal to the MMSE control circuit 5. Supplied with the common error signal from the subtractor 8 and the despread signals from the despreading circuits 31-1 through 3-L-N, the MMSE control circuit 5 controls the antenna weights so that a mean square of the common error signal is minimized.
Herein, the MMSE control circuit 5 controls or updates the antenna weights by the use of an adaptive update algorithm. In order to follow instantaneous channel fluctuation by controlling the antenna weights, a high-speed algorithm, for example, an RLS (Recursive Least Square) algorithm can be used as the adaptive update algorithm.
Description will be made of an operation of the signal received at the m-th symbol (at a time instant mT where T represents a symbol period) through the l-th (l=1 through L) path of the multipath for the k-th user. Herein, the despread signal derived from the signal received by the n-th (n=1 through N) receiving antenna through the l-th path of the multipath is represented by yk,l,n(m). The despread signals yk,l,n(m) are supplied to the antenna weighting/combining circuits 4-1 through 4-L. The multipliers 9-1 through 9-N in the antenna weighting/combining circuits 4-1 through 4-L multiply the despread signals yk,l,n(m) by the antenna weights produced by the MMSE control circuit 5 to produce the weighted signals. The weighted signals are combined by the adder 10. The adder 10 produces the output signal as the antenna combined signal.
Let the antenna weight for the n-th receiving antenna be represented by wk,l,n(m). Then, the antenna combined signal zk,l(m) for the l-th path of the multipath and for the k-th user is given by:
                                                        z                              k                ,                l                                      ⁡                          (              m              )                                =                                    ∑                              n                =                1                            N                        ⁢                                                  ⁢                                                            y                                      k                    ,                    l                    ,                    n                                                  ⁡                                  (                  m                  )                                            ⁢                              w                                  k                  ,                  l                  ,                  n                                            *                              (                m                )                                                    ,                            (        1        )            where * represents a complex conjugate.
The adder 6 in FIG. 1 adds the antenna combined signals produced by the antenna weighting/combining circuits 4-1 through 4-L so that the rake combination is carried out. The rake combined signal zk(m) for the k-th user is given by:
                                          z            k                    ⁡                      (            m            )                          =                              ∑                          l              =              1                        L                    ⁢                                          ⁢                                                    z                                  k                  ,                  l                                            ⁡                              (                m                )                                      .                                              (        2        )            The rake combined signal zk(m) is supplied to the subtractor 8.
In accordance with the RLS algorithm and by the use of all input samples up to the current time instant, the MMSE control circuit 5 controls the antenna weights so that a square sum of exponential weight errors is directly minimized. The square sum {circumflex over (Q)}(m)is represented by:
                                          Q            _                    ⁡                      (            m            )                          =                              ∑                          i              =              1                        m                    ⁢                                          ⁢                                    α                              m                -                i                                      ⁢                                                                                                e                    k                                    ⁡                                      (                    m                    )                                                                              2                                                          (        3        )            Herein, α represents a weighting factor (0<α≦1) and ek(m) represents the common error signal produced by the subtractor 8. As described above, the common error signal is obtained by subtracting the rake combined signal produced by the adder 6 from the reference signal produced by the reference signal producing circuit 7.
The subtractor 8 delivers the common error signal to the MMSE control circuit 6. The common error signal ek(m) is given by:ek(m)={circumflex over (z)}k(m)−zk(m),   (4)where {circumflex over (z)}k(m) represents the reference signal for the k-th user.
In the RLS algorithm, a correlation matrix Rxxk is calculated by a time average of exponential weights according to Equation (3):Rxxk(0)=δU (m=0)   (5)Rxxk(m)=αRxxk(m−1)+Xk(m)XkH(m) (m=1, 2, 3, . . . )   (6)Herein, δ represents a positive constant, H, a complex conjugate transpose, U, a unit matrix. Xk(m) represents a despread signal vector of the despread signal produced by each of the despreading circuits 3-1-1 through 3-L-N and is defined by:
                                                                                          X                  k                                ⁡                                  (                  m                  )                                            =                            ⁢                              [                                                                            y                                              k                        ,                        1                        ,                        1                                                              ⁡                                          (                      m                      )                                                        ,                                                            y                                              k                        ,                        1                        ,                        2                                                              ⁡                                          (                      m                      )                                                        ,                  …                  ⁢                                                                          ,                                                            y                                              k                        ,                        1                        ,                        n                                                              ⁡                                          (                      m                      )                                                        ,                                                                                                                      ⁢                                                                    y                                          k                      ,                      2                      ,                      1                                                        ⁡                                      (                    m                    )                                                  ,                                                      y                                          k                      ,                      2                      ,                      2                                                        ⁡                                      (                    m                    )                                                  ,                …                ⁢                                                                  ,                                                      y                                          k                      ,                      2                      ,                      n                                                        ⁡                                      (                    m                    )                                                  ,                                                                                                                                        ⁢              ⋮                                                                                                          ⁢                                                                                                    y                                                  k                          ,                          L                          ,                          1                                                                    ⁡                                              (                        m                        )                                                              ,                                                                  y                                                  k                          ,                          L                          ,                          2                                                                    ⁡                                              (                        m                        )                                                              ,                    …                    ⁢                                                                                  ,                                                                  y                                                  k                          ,                          L                          ,                          n                                                                    ⁡                                              (                        m                        )                                                                              ]                                            T                                                          (        7        )            Herein, T represents the transpose.
In accordance with the adaptive update algorithm, the MMSE control circuit 5 updates the antenna weights by the use of the common error signal ek(m) produced by the subtractor 8 and the despread signals produced by the despreading circuits 3-1-1 through 3-L-N. In this updating operation, the antenna weights are adaptively controlled by a MMSE criterion so that the common error signal ek(m) is minimized. The updating operation is represented by:
                                          W            k                    ⁡                      (            m            )                          =                                            W              k                        ⁡                          (                              m                -                1                            )                                +                                    γ              k                        ⁢                                          R                xxk                                  -                  1                                            ⁡                              (                                  m                  -                  1                                )                                      ⁢                                          X                k                            ⁡                              (                m                )                                      ⁢                                          e                k                *                            ⁡                              (                m                )                                                                        (        8        )                                          γ          k                =                  1                      α            +                                                            X                  k                  H                                ⁡                                  (                  m                  )                                            ⁢                                                R                  xxk                                      -                    1                                                  ⁡                                  (                                      m                    -                    1                                    )                                            ⁢                                                X                  k                                ⁡                                  (                  m                  )                                                                                        (        9        )            
Herein, Wk(m) represents an antenna weight vector of the antenna weight produced by the MMSE control circuit 5 and is defined by:
                                                                                          W                  k                                ⁡                                  (                  m                  )                                            =                            ⁢                              [                                                                            w                                              k                        ,                        1                        ,                        1                                                              ⁡                                          (                      m                      )                                                        ,                                                            w                                              k                        ,                        1                        ,                        2                                                              ⁡                                          (                      m                      )                                                        ,                  …                  ⁢                                                                          ,                                                            w                                              k                        ,                        1                        ,                        n                                                              ⁡                                          (                      m                      )                                                        ,                                                                                                                      ⁢                                                                    w                                          k                      ,                      2                      ,                      1                                                        ⁡                                      (                    m                    )                                                  ,                                                      w                                          k                      ,                      2                      ,                      2                                                        ⁡                                      (                    m                    )                                                  ,                …                ⁢                                                                  ,                                                      w                                          k                      ,                      2                      ,                      n                                                        ⁡                                      (                    m                    )                                                  ,                                                                                                                                        ⁢              ⋮                                                                                                          ⁢                                                                                                    w                                                  k                          ,                          L                          ,                          1                                                                    ⁡                                              (                        m                        )                                                              ,                                                                  w                                                  k                          ,                          L                          ,                          2                                                                    ⁡                                              (                        m                        )                                                              ,                    …                    ⁢                                                                                  ,                                                                  w                                                  k                          ,                          L                          ,                          n                                                                    ⁡                                              (                        m                        )                                                                              ]                                            T                                                          (        10        )            
Equations (8) and (9) require the calculation of an inverse matrix Rxxk−1 of the correlation matrix Rxxk. The inverse matrix Rxxk−1 is calculated by the use of a matrix formula:Rxxk−1(0)=δ−1U (m=0)   (11)
                                                        R              xxk                              -                1                                      ⁡                          (              m              )                                =                                                    1                α                            ⁢                                                R                  xxk                                      -                    1                                                  ⁡                                  (                                      m                    -                    1                                    )                                                      -                                                                                R                    xxk                                          -                      1                                                        ⁡                                      (                                          m                      -                      1                                        )                                                  ⁢                                                      X                    k                                    ⁡                                      (                    m                    )                                                  ⁢                                                      X                    k                    H                                    ⁡                                      (                    m                    )                                                  ⁢                                                      R                    xxk                                          -                      1                                                        ⁡                                      (                                          m                      -                      1                                        )                                                                                                α                  2                                +                                  α                  ⁢                                                                          ⁢                                                            X                      k                      H                                        ⁡                                          (                      m                      )                                                        ⁢                                                            R                      xxk                                              -                        1                                                              ⁡                                          (                                              m                        -                        1                                            )                                                        ⁢                                                            X                      k                                        ⁡                                          (                      m                      )                                                                                                          ⁢                                  ⁢                  (                                    m              =              1                        ,            2            ,            3            ,            …                    )                                    (        12        )            
However, the above-mentioned technique is disadvantageous in the following respects. The MMSE control circuit 5 requires a large amount of calculation according to the adaptive update algorithm. Such a large amount of calculation imposes a large processing load upon a digital signal processor (DSP). This is because, since the common error signal is used, the adaptive update algorithm for controlling the antenna weights so as to minimize the mean square of the common error signal requires calculation of an (N×L)-order correlation matrix Rxxk.